The present application describes the addition of a common mode (CM) choke to a balanced amplifier that reinforces the balanced differential mode signal while rejecting the common mode signals, such as even order harmonics.
Balanced amplifiers, such as balanced push pull amplifiers, can be used to provide suppression of second and other even order harmonics. A balanced amplifier with low levels of imbalance (i.e., phase and amplitude imbalance between the in-phase and anti-phase sides of the amplifier) exhibits reduced even harmonics. The suppression of the even harmonics can be reduced if the amplifier imbalances increase. Cascading extra amplifier stages can increase amplitude and phase imbalance, as can component variations caused by high volume manufacturing.
Manufacturers try to achieve high levels of amplitude and phase balance in balanced amplifiers by using components that are inherently well balanced (i.e., components with very low levels of amplitude and phase imbalance), using select test or tuning methods to improve amplitude and phase balance or using a tightly controlled symmetrical layout of the printed circuit board (PCB) and other components. A common solution is to introduce two balancing/unbalancing transformers (baluns) between each cascaded amplifier stage of the balanced amplifier to limit the imbalances and reduce the even harmonics. However, the addition of extra baluns has the disadvantage of resulting in the loss of gain, reduced power efficiency and increased cost.